Hydraulic control system of automatic transmission for automotive vehicle

Abstract

A hydraulic control system of an automatic transmission for an automotive vehicle includes a hydraulic pump, a pressure regulating valve, a torque converter, a converter clutch, first, second and third solenoid valves, first through eighth friction elements, second and third speed clutch valves, a band valve and a manual value. The pressure regulating valve varies a hydraulic pressure when shifting, and the converter clutch regulator valve actuates a damper clutch to improve transmission efficiency of the torque converter. The first, second and third solenoid valves supply or obstruct the supply of hydraulic pressure from the pressure regulating valve to a respective one of a first, second and third clutch shift valve to displace valve spools of a respective one of the first, second and third clutch shift valves. The second speed and third speed clutch shift valves as well as the fourth speed band valve, selectively supply hydraulic pressure to the friction elements to perform shifting. The second speed and third speed clutch shift valves as well as the fourth speed band valve are controlled by the solenoid valve and an overdrive unit valve.

What is claimed is: 
     
       1. A hydraulic control system of an automatic transmission for an automotive vehicle, comprising: a hydraulic pump, driven by an engine, for pressurizing fluid;   a pressure regulating valve for varying a hydraulic pressure when shifting to a forward mode or a reverse mode;   a torque converter for transmitting a power of the engine to an input shaft of the transmission;   a converter clutch regulator valve for actuating a damper clutch to improve a transmission efficiency of said torque converter;   first, second and third solenoid valves controlled by a transmission control unit for supplying the hydraulic pressure or obstructing a supply of the hydraulic pressure which is varied in said pressure regulating valve to shift valves of a transmission control section to displace valve spools of said shift valves;   a first to a eighth friction elements commonly actuated at each speed stage;   second speed and third speed clutch valves, a fourth speed band valve and an overdrive unit valve controlled by ON/OFF functions according to duty ratios of said solenoid valves for selectively supplying the hydraulic pressure to said friction elements to perform shifting; and   a manual valve operated by a shift select lever, for supplying the hydraulic pressure to a reverse pressure conduit or to a forward pressure conduit.   
     
     
       2. The hydraulic control system as claimed in claim 1, wherein said second speed clutch valve is controlled according to the duty ratio of said first solenoid valve and open/close states of at least two ports of said second speed clutch valve are changed thereby to make said second friction element actuated by an action of the hydraulic pressure of said forward pressure conduit and to supply the hydraulic pressure to said third speed clutch valve for a next speed stage. 
     
     
       3. The hydraulic control system as claimed in claim 1, wherein said third speed clutch valve is controlled according to the duty ratio of said second solenoid valve and open/close states of at least two ports of said third speed clutch valve are changed thereby to make said third friction element actuated and to supply the hydraulic pressure to said fourth speed band valve for a next speed stage. 
     
     
       4. The hydraulic control system as claimed in claim 1, wherein said fourth speed band valve is controlled according to the duty ratio of said third solenoid valve to make said fourth friction element actuated and to supply the hydraulic pressure to an overdrive unit valve for a next speed stage. 
     
     
       5. The hydraulic control system as claimed in claim 1, wherein said second solenoid valve is controlled according to a low duty ratio in a second speed to open a hydraulic passage to said third friction element and said third and fourth solenoid valves are controlled according to high duty ratios to open hydraulic passages to said fourth and first friction elements respectively to perform a skip shifting to a fifth speed. 
     
     
       6. The hydraulic control system as claimed in claim 1, wherein a first port of said second speed clutch valve is connected to a third port which can be used to displace a valve spool of a reverse clutch inhibiter valve whose ports are opened or closed according to an ON/OFF operation of said fifth solenoid valve.